Support using a PlantUML server

[rose-a]

It would be nice if was possible to use a PlantUML Server to parse the UML stuff instead of installing it locally with all its dependencies (first integration in this way seen at GitLab).

This would make it very easy to use it in combination with a CI server, regardless of the operating system of the development machine and the build server.

[koppor]

Maybe you find the appropriate place in https://github.com/latextemplates/plantuml/blob/master/plantuml.lua. I think, one could do magic with environment variables?

[koppor]

In other settings, we get PlantUML in place using following command:

RUN wget https://netix.dl.sourceforge.net/project/plantuml/1.2018.2/plantuml.1.2018.2.jar -q --output-document=/home/plantuml.jar
ENV PLANTUML_JAR=/home/plantuml.jar

Source: https://github.com/dante-ev/docker-texlive/blob/8c2509cc8fa445c00c22d2e9800ed675cdce3791/Dockerfile#L66